YUCCA utilities and other Cydia Substrate fixes are currently unstable for iOS 10/10.1.1 jailbreak and must be avoided.

iOS 10.1.1 jailbreakers have been repeatedly warned by Italian hacker and jailbreak developer, Luca Todesco, to avoid installing the YUCCA Utilities package or any other Cydia Substrate enabler as the feature is apparently broken and could brick your device easily. Although the YUCCA tool was meant to fix Cydia crashing issues and low-storage warnings, it has reportedly caused widespread boot-fail issues and system stability problems.

Consequently, prospective jailbreakers are advised to hold off any temptation of installing the YUCCA tool and instead wait for Todesco to release the iOS 10.2 jailbreak. Those who have soft-bricked their device may try performing a system restore with fresh/clean files of iOS 10.2.

Here is what Saurik (the founder of Cydia) had to say regarding the use of YUCCA tool with iOS 10.1.1 jailbreak:

"I am concerned the YUCCA package is so broken the only real recommendation I have is to restore your device to 10.2 (though I'd also recommend waiting until Luca releases a jailbreak and just hope that when that happens iOS 10.2 is still a restorable option).

"As for Substrate, I wouldn't worry too much about anything: I'm pretty certain your "worst case scenario" is to hold down volume up while running a new mach_portal, which will disable Substrate entirely, at which point you can fix any issues you have via Cydia."

As a word of advice, Saurik has suggested putting the device into 'No Substrate mode' and then fix prevalent issues with Cydia, after jailbreaking the device on iOS 10/10.1.1. Those who are yet to jailbreak are recommended to wait until the first public jailbreak for iOS 10.2 is rolled out by Todesco as the hacker has recently confirmed his latest progress with the impending jailbreak for iOS 10.2 via Twitter.